Flammability Characteristics of Thermally Modified Oak ......Flammability Characteristics of Thermally Modified Oak Wood Treated with a Fire Retardant Miroslav Gašparík,a,* Linda

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Gašparík et al. (2017). “Burning thermally mod. oak,” BioResources 12(4), 8451-8467. 8451

Flammability Characteristics of Thermally Modified Oak Wood Treated with a Fire Retardant

Miroslav Gašparík,a,* Linda Makovická Osvaldová,b Hana Čekovská,a and

Dalibor Potůček a

Flammability characteristics were determined for oak wood (Quercus robur L.), which was thermally modified at 160, 180, and 210 °C. Subsequently, the thermally modified and unmodified wood was treated with a fire retardant. The effect of the thermal modification (TM) and fire retardant treatment (FRT) on the weight loss (WL), burning rate (BR), maximum burning rate (MBR), and time to reach the maximum burning rate (TRMBR) were evaluated. The FRT had an expected positive effect on all of the flammability characteristics, where the WL, BR, and MBR decreased, and the TRMBR increased. The TM temperature did not have a clear effect. As the TM temperature increased, the WL and BR decreased. The highest differences were found at 160 and 180 °C. As the TM temperature increased for the wood without the FRT, the TRMBR decreased. During the burning of the thermally modified wood with the FRT, the trend was the exact opposite.

Keywords: Burning rate; Maximum burning rate; Weight loss; Thermal modification; Fire retardant; Oak

Contact information: a: Department of Wood Processing, Czech University of Life Sciences in Prague,

Kamýcká 1176, Praha 6- Suchdol, 16521 Czech Republic; b: Department of Fire Engineering, Faculty of

Security Engineering, University of Žilina, Univerzitná 1, Žilina, 010 26 Slovakia;

* Corresponding author: [email protected]

INTRODUCTION

Wood is a natural material with many excellent properties (aesthetic and

mechanical) that make it suitable for versatile use. Despite those properties, wood has

certain disadvantages, such as a low resistance to biological agents (fungi, insects, etc.),

changes to the physical and mechanical properties, and high flammability. A common

method of protecting wood against most of these effects is thermal modification (TM).

TM is a newer type of wood modification. There are currently several types of

TM, the principle of which is different depending on the medium and temperature used

(Navi and Sandberg 2012). The most well-known types of TM are ThermoWood in

Finland, Plato Wood in the Netherlands, oil-heat treatment (OHT) in Germany, and Les

Bois Perdure and rétification process (Retiwood) in France (Esteves et al. 2011;

Sandberg and Kutnar 2016). In addition to these common TMs, there are also lesser

known TM processes that use superheated steam, such as Wood Treatment Technology

(WTT) in Denmark and Firmolin technology in the Netherlands, or a partial vacuum, like

Termovuoto in Italy (Ferrari et al. 2013). In general, TM can be defined as a process in

which high temperatures ranging between 150 and 260 °C are applied to wood in an

environment with different types of media (steam, nitrogen, oil, etc.) without chemical

substances (Sandberg and Kutnar 2016). TM remarkably affects the basic components of

wood. Cellulose and hemicellulose are polysaccharides found in the cell walls of wood.



Cellulose, which is responsible for the strength of wood fibers because of its high

polymerization degree, is more resistant to heat (Pandey 1999). Hemicellulose is less

resistant to heat, and its degradation leads to the most significant decrease in the

mechanical properties of thermally modified wood (Osvald and Gaff 2017). Lignin has

an aromatic structure and fills the spaces between the lignocellulosic fibers. It is

responsible for the rigidity of the material and is the most resistant to heat of these three

components (Cademartori et al. 2013). TM improves the dimensional stability, durability,

hygroscopicity, weather resistance, and resistance to fungi (Jämsä et al. 2000; Candelier

et al. 2013; Kačíková et al. 2013; Baysal et al. 2014; Kondratyeva et al. 2016). In

contrast, TM, especially above 220 °C, noticeably reduces the mechanical properties, e.g.

the modulus of rupture, hardness, and impact bending strength (Kamdem et al. 2002;

Kačík et al. 2015).

Mechanical properties are, however, only one group that is important for wood

products. Fire properties are also important because fire is one of the most critical

physical degradation factors for wood (Lee et al. 2004; Mohebby et al. 2007; Jiang et al.

2010). The fire properties of common wood-based materials have been thoroughly

explored. Although there are several studies that deal with the flammability and burning

of thermally modified wood (Martinka et al. 2013; Xing and Li 2014; Čekovská et al.

2017a; Čekovská et al. 2017b), their fire properties have only been investigated to a

limited extent. Thermally modified wood is susceptible to flames and burning

(Delichatsios et al. 2003), but is still used for interior and exterior elements. TM releases

volatile components from the wood and degrades lignin and polysaccharides, which are

closely related to the flammability and burning properties of wood (Manninen et al.

2002). Hence, thermally modified wood contains less volatile flammable components,

and the likelihood of it igniting and subsequently combusting is reduced (Martinka et al.

2013). The flammability of wood can be modified with fire retardants (Sogutlu et al.

2011; Lowden and Hull 2013).

Wood fire retardants affect the burning of wood by delaying the ignitability and

flame spread, and reducing the heat release (Östman et al. 2001; Hagen et al. 2009). Fire

retardants work on different principles, although most of them use several mechanisms at

once (Pries and Mai 2013). Most often, fire retardants create non-combustible charcoal

layers on the surface of the wood under the influence of temperature and physically

inhibit oxygen access to the wood (Jiang et al. 2014; Merk et al. 2015). Fire retardants

for wood use chemicals based on phosphorus, nitrogen, boron, and combinations of these

elements (Wang et al. 2004; Gao et al. 2006; Marney et al. 2008). Fire retardants are

applied through various methods. Vacuum pressure impregnation is highly effective

because it gets the fire retardant into a considerable depth, but it is only suitable for

structural elements, facing, and flooring materials before they are used. Soaking does not

require complicated machinery and is suitable for fire retardants that are in solutions that

can penetrate into wood. Coatings, which are suitable for existing structures and built-in

elements, use coating substances that form an insulating layer after they dry.

This research studied the flammability characteristics of thermally modified and

unmodified oak wood. Each group of samples was divided into two subgroups according

to their fire retardant modification. A two-component Flamgard Transparent coating was

used as the fire retardant. A burning test was used to evaluate the basic characteristics,

including the weight loss (WL), burning rate (BR), maximum burning rate (MBR), and

time to reach the maximum burning rate (TRMBR).



EXPERIMENTAL Materials

Seventy-five-year-old pedunculate oak (Quercus robur L.), used in this study,

was harvested from the Poľana region in central Slovakia, near Zvolen. Wood samples

with the dimensions 20 × 100 × 200 mm were cut from the section located in the middle

distance between pith and bark (Fig. 1). The samples were divided into two groups: the

first group was thermally modified, while the second group was unmodified. Both groups

contained two subgroups, one of which consisted of fire retardant-treated samples, while

the other one contained samples that did not undergo the fire retardant treatment (FRT).

All of the samples were dried in a drying oven ULM 400 (Memmert GmbH & Co. KG,

Schwabach, Germany) at a temperature of 103 ± 2 °C until an oven-dry state was

reached. This condition facilitated the TM, and also induced similar moisture contents in

the samples without TM, which was important for comparison purposes.

Fig. 1. Oak samples

Procedures Thermal modification (TM)

The wood samples were modified in a thermal chamber (S400/03, LAC Ltd.,

Rajhrad, Czech Republic). Three final temperatures, 160, 180, and 210 °C, were chosen

for the wood modification. The TM used the ThermoWood principle developed by VTT

(Finland), which takes place in a protective atmosphere, dispersed water in the air, to

prevent overheating and burning. The TM of the wood was carried out in three basic

phases (Table 1).

Table 1. Conditions and Parameters of the Thermal Modification

Thermal Modification (TM) Temperature

160 °C 180 °C 210 °C

Heating 10.6 h 11.4 h 14.6 h

Thermal modification 3 h 3 h 3 h

Cooling 6.4 h 7.2 h 8 h

Total time 20 h 21.6 h 25.6 h



The first phase of the TM consisted of heating the wood. During this phase, the

temperature gradually increased until the desired temperature (160, 180, and 210 °C) was

reached. In the second phase, the final temperature was maintained for 3 h. This time was

the same for all of the TM temperatures. The third phase was characterized by a gradual

cooling. The temperature was gradually reduced to 40 °C so that the wood did not

undergo temperature and humidity shock when the chamber was opened. Then, the

samples rested for 3 h in the ambient environment. The densities of the samples are listed

in Table 2.

Table 2. Density and Moisture Content of the Oak Samples

Unmodified Thermal Modification (TM) Temperature

160 °C 180 °C 210 °C

Density before TM (kg/m3) 774 769 766 775

Density after TM (kg/m3) 774 736 683 673

Original moisture content (%) 10.5 10.8 10.7 11.2

Moisture content after TM (%) 5.5 5.8 4.9 4.2

Fire retardant treatment (FRT)

A two-component fire retardant coating from Flamgard Transparent (Stachema

a.s., Rovinka, Slovakia) was used. The first component is a foamable, water-soluble

coating substance (a mixture consisting of ammonium phosphates, a foam forming agent,

oxalic and acetic acid, and fire retardant additives) that was applied in three layers so that

the total spread was at least 500 g/m2. The second component is an acrylic cover lacquer

S 1818, which was applied in a single layer with a spread of at least 80 g/m2. Both

components were applied with a brush according to the conditions specified by the

manufacturer (Table 3). The curing of the individual layers took place at 20 °C.

Table 3. Properties and Application Conditions of the Fire Retardant Treatment

Component pH Density Thinner

Type

Wood Moisture Content

Content of Non-volatile Components

Application Ambient

Temperature

Hardening Time

Coating substance

2 to 3 1.2 g/cm3 hot water 10% min. 62% 10 to 35 °C 12 h/layer*

Cover lacquer

- 0.97 g/cm3 C 6000 - 31.5% 20 °C 4 h

* last layer must dry for 24 h

Methods Determination of the flammability characteristics

The flammability test was based on the direct application of flame to the wood,

according to ČSN 73 0862/B-2 (1991). In this study, the weight was measured

continuously during the test. Therefore, the sample holder and stand were placed on a

scale to measure the WL throughout the burning process (Fig. 2). A sample was placed in

the holder at a 45° angle to the horizontal plane with the assessed surface facing down.

The height of the propane burner flame was set to 10 cm. The burner was placed at the

center of the sample (intersection of the diagonals) so that the distance between the

mouth of the burner and center of the sample was 9 cm. Weight measurements were



taken at 10-s intervals using a laboratory scale (MS 1602S, Mettler Toledo, Greifensee,

Switzerland). The whole test lasted for 10 min (Fig. 3). In addition to the WL, the BR,

MBR, and TRMBR were also determined. Each combination of TM temperature and

FRT was represented by five samples according to ČSN 73 0862/B-2 (1991), so the

whole study contained 40 samples.

Fig. 2. Burning test apparatus

Fig. 3. Oak sample during the burning test

Evaluation and calculation

The effect of the factors on the flammability characteristics were assessed with

Statistica 13 software (Statsoft Inc., Tulsa, OK, USA) by analysis of variance (ANOVA).

The wood density was determined before and after the TM according to ISO

13061-2 (2014). The moisture content was determined according to ISO 13061-1 (2014).

Drying to an oven-dry state was also carried out according to ISO 13061-1 (2014).

The WL was calculated according to ČSN 73 0862/B-2 (1991) with Eq. 1,

1000

10

m

mmm (1)

where m is the WL (%), m0 is the weight of the sample before the test (g), and m1 is the

weight of the sample at a certain time interval (every 10 s as well as after 10 minutes) (g).



The burning rate was calculated with Eq. 2,

10010

0

10

t

tt

m

mmv (2)

where v is the burning rate (%/s), mt is the weight of the sample at time t (g), mt+10 is the

weight of sample 10 s later (g), and mt0 is the weight of sample at time 0 (g).

RESULTS AND DISCUSSION

The statistical evaluation of the effects from the factors and combination of

factors on the flammability characteristics of the oak wood are listed in Table 4.

Table 4. Effect of the Factors on the Flammability Characteristics

Monitored Factor Sum of Squares

Degrees of Freedom

Variance Fisher's F - Test

Significance Level P

Weight Loss

Intercept 3,246.213 1 3,246.213 1,045.588 0.000000*

Fire retardant treatment (FRT)

707.575 1 707.575 227.906 0.000000*

Thermal modification (TM) 110.173 3 36.724 11.829 0.000022*

FRT × TM 114.298 3 38.099 12.272 0.000017*

Error 99.350 32 3.105

Burning Rate

Intercept 3,995.002 1 3,995.002 129.1936 0.000000*

FRT 837.683 1 837.683 27.0896 0.000011*

TM 157.731 3 52.577 1.7003 0.186646

FRT × TM 318.824 3 106.275 3.4368 0.028347*

Error 989.523 32 30.923

Maximum Burning Rate

Intercept 44,906.75 1 44,906.75 518.4761 0.000000*

FRT 11,060.61 1 11,060.61 127.7016 0.000000*

TM 1,604.85 3 534.95 6.1763 0.001963*

FRT × TM 467.08 3 155.69 1.7976 0.167491

Error 2,771.62 32 86.61

Time to Reach the Maximum Burning Rate

Intercept 871,725.6 1 871,725.6 130.5345 0.000000*

FRT 20,475.6 1 20,475.6 3.0661 0.089524

TM 16,686.9 3 5,562.3 0.8329 0.485707

FRT × TM 31,036.9 3 10,345.6 1.5492 0.220857

Error 213,700.0 32 6,678.1

* Statistically significant (P < 0.05) factors or its combination



Weight Loss The WL is a factor that directly represents the wood flammability, i.e. the greater

the WL of the wood, then the higher its flammability. A statistical evaluation of the effect

of the TM on the WL is shown in Fig. 4. On average, the WL of the thermally

unmodified wood was 7.5%. The thermally modified wood exhibited the highest WL of

11.6% at the lowest temperature of 160 °C, and the WL gradually decreased as the

temperature increased. The wood thermally modified at 210 °C had a WL identical to that

of the unmodified wood.

Fig. 4. Effect of the TM on the WL

The effect of the fire retardant (Fig. 5) was significant, and as was expected, the

WL was lower for the wood with the fire retardant coating. The fire retardant reduces the

burning capacity of wood, which logically suggests that the WL of the wood should be

lower. The wood without the FRT had an average WL of 13.2%, while the wood with the

FRT had a WL of only 4.8%, which was a decrease of 63.6%.

Fig. 5. Effect of the FRT on the WL

The interaction between the FRT and TM is shown in Fig. 6. The wood without

the FRT showed more pronounced differences in the WL values found for the treated

wood. The highest WL values for the wood without the FRT were found for the wood



modified at 160 and 180 °C. In this case, it is interesting to note that the average WL of

the unmodified wood was 9.9%, and for the wood thermally modified at 210 °C, it was

10.5%, which was a small difference.

Fig. 6. Effect of the TM and FRT on the WL

In the case of the wood with the FRT, smaller differences were observed at the

different TM temperatures. As with the previous case, the highest WL was measured at

160 °C, but the lowest WL was achieved with the wood thermally modified at 180 °C.

Burning Rate

The BR represents the spread of fire on a material over a certain time, i.e. a higher

BR means a better flammability of the material. In this study, the effect of the TM on the

BR was not clear (Fig. 7). The highest BR (13.2 × 10-5 %/s) was measured in the wood

thermally modified at 180 °C.

Fig. 7. Effect of the TM on the BR

In contrast, the unmodified wood achieved a 30.7% lower BR compared with the

wood thermally modified at 180 °C. The BR values of the wood thermally modified at

160 and 210 °C were almost the same. Čekovská et al. (2017a), who investigated the

effect of TM on the burning characteristics of spruce wood, found a similar effect from



the modification temperature on the BR.

The FRT (Fig. 8) had the same effect on the BR as it did on the WL, i.e. the wood

with the applied fire retardant had a lower BR (5.4 × 10-5 %/s) than that of the wood

without the FRT (14.6 × 10-5 %/s). The BR decreased by 63% because of the fire

retardant application. Fire retardants are specially designed to reduce burning, so this

effect on the BR was expected. The previous works by Marney et al. (2008), Lowden and

Hull (2013), and Jiang et al. (2014) confirmed this effect of fire retardants on various fire

characteristics of wood.

Fig. 8. Effect of the FRT on the BR

The thermally modified wood without the FRT exhibited great differences in

the BR at individual TM temperatures (Fig. 9). The highest BR was found at 180 °C,

while the lowest BR was found at 160 °C.

Fig. 9. Effect of the TM and FRT on the BR

The wood thermally modified at 210 °C achieved a BR similar to that found for

the unmodified wood. The FRT had a clear effect on the BR. After the application of the

FRT, these differences were reduced and the BR was dependent on the TM temperature,

i.e. the BR decreased as the modification temperature rose.



Maximum Burning Rate The dependence of the MBR on the TM temperature (Fig. 10) was very similar to

that found for the BR. A significant difference was evident in the wood thermally

modified at 160 °C, where the MBR values were the highest (44.3 × 10-5 %/s), which was

the opposite of the trend seen for the BR (Fig. 7). The MBR value for the wood thermally

modified at 180 °C was 9.5% higher than that of the wood thermally modified at 210 °C.

The MBR value of the wood thermally modified at 210 °C was close to the value

achieved for the unmodified wood, with a difference of only 1.7%.

Fig. 10. Effect of the TM on the MBR

In the case of the MBR, the FRT (Fig. 11) also had a significant effect on its

values. As for the previously discussed properties, the wood with the FRT achieved

significantly lower MBR values.

Fig. 11. Effect of the FRT on the MBR The FRT reduced the MBR by 66.3%, which was similar to the decrease seen for

the BR.

For the effects of the FRT and TM on the MBR (Fig. 12), there were differences

compared with the BR. The wood without the FRT reached the highest MBR values at a

TM temperature of 160 °C, which was the opposite of the trend observed for the BR (Fig.



9). For the wood with the FRT, the highest MBR value was achieved at a TM

temperature of 160 °C, and the lowest value was achieved at 180 °C.

Fig. 12. Effect of the TM and FRT on the MBR

Time to Reach the Maximum Burning Rate The TRMBR is a MBR-dependent factor, but has an opposite trend, i.e. the

higher the TRMBR, the slower the material burns. The TRMBR was strongly affected by

the TM temperature (Fig. 13). As the temperature increased, the TRMBR values

gradually decreased. The only significant change occurred at a TM temperature of 160

°C, where the TRMBR was significantly lower (120 s) compared with the other

temperatures.

Fig. 13. Effect of the TM on the TRMBR

As was expected, the FRT had a strong effect on the TRMBR (Fig. 14). The

higher the fire retardant ability, the higher the TRMBR. The wood treated with a fire

retardant achieved TRMBR values that were 26.6% higher than that of the untreated

wood.



Fig. 14. Effect of the FRT on the TRMBR

Generally, the TRMBR was dependent on both the TM temperature and FRT

(Fig. 15 and Table 5). The application of a fire retardant caused the thermally modified

wood to achieve higher TRMBR values as the temperature increased for almost all of the

samples, except for the wood modified at 160 °C. In contrast, the wood without the FRT

showed the exact opposite behavior, i.e. as the TM temperature increased, the TRMBR

significantly decreased. The TRMBR for the wood without TM was 188 s, while the

wood thermally modified at 210 °C only reached 86 s, which was a decrease of 54.3%.

Čekovská et al. (2017a) found significantly smaller differences in the TRMBR values in

relation to the increase in the TM temperature for spruce wood.

Fig. 15. Effect of the TM and FRT on the TRMBR

The results in Table 5 show that in some cases there was a higher weight loss, but

the burning rate was the lowest. The relationship between BR and WL was not directly

proportional. WL represents the total loss of weight relative to the original weight of

sample, but does not describe how fast each proportion of wood was burnt over a 10-

second measuring interval. BR is influenced not only by the amount of the burnt mass of



wood, but also by the proportion of its basic constituents found there. Higher proportion

of hemicelluloses mean faster burning, but lower weight loss. Conversely, a higher

proportion of cellulose and lignin means slower burning, but higher weight loss. These

proportions are also influenced by the thermal modification, which influences the various

components differently. Hemicellulose is the least resistant to temperatures and its

decomposition is up to 200 °C. Above this temperature, only cellulose and lignin remain,

but they burn more slowly than hemicellulose.

Table 5. Average Values of the Flammability Characteristics

Factors Flammability Characteristics

Fire Retardant Treatment

Thermal Modification

(°C)

Weight Loss (%)

Burning Rate

(× 10-5 %/s)

Maximum Burning

Rate (× 10-5 %/s)

Time to Reach Maximum

Burning Rate (s)

without FRT unmodified (reference)

9.9 (22.8) 13.2 (16.1) 41.7 (14.5) 188 (2.8)

without FRT 160 16.4 (16.3) 9.9 (14.8) 62.9 (3.5) 114 (7.8)

without FRT 180 16.0 (13.5) 22.4 (18.6) 52.9 (5.9) 112 (2.3)

without FRT 210 10.5 (10.7) 12.8 (6.9) 43.0 (1.5) 86 (17.6)

with FRT unmodified (reference)

5.1 (4.4) 7.1 (3.5) 16.8 (5.4) 160 (8.8)

with FRT 160 6.8 (4.7) 6.4 (3.3) 25.7 (2.3) 125 (3.6)

with FRT 180 2.6 (20.4) 3.9 (6.7) 10.6 (3.6) 204 (6.8)

with FRT 210 4.7 (12.5) 4.3 (6.2) 14.4 (3.9) 192 (4.8)

The values in parentheses are the coefficients of variation (CV, %).

Thermally modified wood has properties that have been changed, which affect its

ignition and subsequent burning behavior. The properties of thermally modified wood

depend not only on the wood species and its properties, but also on the modification

method and final temperature used during modification. Under high temperatures, the

three basic components of wood begin gradually decompose, which produces volatile

gases, tar, and carbonaceous char (Lowden and Hull 2013). However, each component

has a different resistance. TM up to 210 °C (the highest temperature in this study) affects

all of the basic wood components, but hemicellulose pyrolysis is the most pronounced

process because it begins at 180 °C (Kim et al. 2006). High temperatures cause the

degradation of cell wall polymers, which results in a reduction in volatile pyrolysis

products and reduces the flammability of the wood (Hirata et al. 1991; Kol and Sefil

2011). From this perspective, thermally modified wood should be more suitable in

regards to fire safety. In contrast, depending on the TM temperature, the surface integrity

of the thermally modified wood is disrupted to a certain extent, which leads to easier

ignition. The surface application of fire retardants is therefore suitable for thermally

modified wood.

In general, wood treated with fire retardants requires a higher temperature and

longer exposure to flames to burn. When the wood is exposed to flames, it releases

pyrolysis gases, which, however, contains less combustible gases needed for the wood to

burn (Hagen et al. 2009). Flame retardants based on phosphorus are the most common

fireproof chemicals for wood (Schartel 2010). Their advantage is that they directly affect



the thermal degradation of wood. During wood pyrolysis, the phosphorus components

produce acids that reduce the decomposition temperature. This effect results in char

formation and the dehydration of wood components (George and Susott 1971; Stevens et

al. 2006). With this char layer on the surface, wood can resist further flame exposure

because this layer increases the thermal resistance between the underlying wood and

pyrolysis. The result is a reduction in the heat release rate and creation of a barrier against

the release of volatiles from the wood and their subsequent mixing with oxygen in the air

(Yang et al. 2003). Additionally, it is assumed that phosphorus prevents the formation of

flammable components (e.g. tar = levoglucosan) by the phosphorylation of cellulose.

The information available on the flammability and burning of thermally modified

wood with or without FRT is not extensive, and therefore it is important to study it

further.

CONCLUSIONS

1. The WL did not have a clear dependence on the TM temperature, whereas the FRT

had a clear effect. The highest WL values were found for the wood thermally

modified at 160 and 180 °C, which were 16.4% and 16%, respectively. However, the

WL observed in the wood thermally modified at 210 °C (9.9%) was comparable to

the WL measured for the unmodified wood (10.5%). The differences in the wood

with the FRT were much lower. As the TM temperature increased, the WL decreased,

except for the wood modified at 160 °C.

2. The BR of the thermally modified wood without the FRT had different values

depending on the TM temperature. The lowest BR was achieved at a TM temperature

of 160 °C, and the highest BR was found for the wood modified at 180 °C. The BR of

the wood modified at 210 °C was comparable to that of the unmodified wood. In

contrast, the FRT application on the thermally modified wood resulted in the

equalization of the differences and a gradual decrease in the BR of up to 38.1% as the

modification temperature increased.

3. The MBR of the thermally modified wood with or without FRT had a similar

dependence on the TM temperature, i.e. the highest MBR was found at a TM

temperature of 160 °C. As the TM temperature increased further, the MBR decreased.

The MBR values for the thermally modified wood ranged from 10.6 to 26.7 × 10-5

%/s, and for the thermally modified wood without the FRT, the MBR values ranged

from 41.7 to 62.9 × 10-5 %/s.

4. The TRMBR of the thermally modified wood without the FRT decreased

significantly as the TM temperature increased. The unmodified wood had an average

TRMBR value of 188 s, but when the temperature was 160, 180, and 210 °C, it

dropped to 114, 112, and 86 s, respectively. The FRT caused the TRMBR to increase

to 192 and 204 s for the wood modified at 180 and 210 °C, respectively. For the

modification at 160 °C, a decline in the TRMBR was observed.



ACKNOWLEDGEMENTS

This research was supported by the University-wide Internal Grant Agency of the

Faculty of Forestry and Wood Science at the Czech University of Life Sciences Prague,

project CIGA 2016-4309.

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Article submitted: July 5, 2017; Peer review completed: September 10, 2017; Revised

version received: September 22, 2017; Accepted: September 23, 2017; Published:

September 26, 2017.

DOI: 10.15376/biores.12.4.8451-8467

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Flammability Characteristics of Thermally Modified Oak ......Flammability Characteristics of Thermally Modified Oak Wood Treated with a Fire Retardant Miroslav Gašparík,a,* Linda